Metal-treating process

ABSTRACT

A process for forming a phosphate coating on metal surfaces wherein an alkali metal and/or ammonium orthophosphate solution, having a pH within the range of about 3.0 to 5.5 is sprayed on the metal surface at a pump pressure of from about 10 to 75 atmospheres. The process carried out in this manner is effective in both cleaning and coating the metal surfaces to which it is applied. In addition to the alkali metal and/or ammonium orthophosphate, the phosphating solution may also contain a benzoate, an hydroxyl amine, a molybdate, and a surface-active agent.

United States Patent [72] lnventors Friedrich Dittel Mainz-Kostheim;

Peter Jorns, Frankfurt am Main; Walter Stenger, Frankfurt am Main, allof Germany [21] Appl. No. 11,901

[22] Filed Feb. 16, 1970 [45] Patented Oct. 26, 1971 [73] AssigneeHooker Chemical Corporation Niagara Falls, N.Y.

[32] Priority Aug. 19, 1969 [3 3] Germany [54] METAL-TREATING PROCESSPrimary Examiner-Ralph S. Kendall Attorneys-Stanley l-l. Lieberstein andWilliam J. Schramm ABSTRACT: A process for forming a phosphate coatingon metal surfaces wherein an alkali metal and/or ammonium orthophosphatesolution, having a pH within the range of about 3.0 to 5.5 is sprayed onthe metal surface at a pump pressure of from about 10 to 75 atmospheres.The process carried out in this manner is effective in both cleaning andcoating the metal surfaces to which it is applied. In addition to thealkali metal and/or ammonium orthophosphate, the phosphating solutionmay also contain a benzoate, an hydroxyl amine, a molybdate, and asurface-active agent.

METAL-TREATING PROCESS This invention relates to a method for thechemical surface treatment of metal, and more particularly it relates toa process for treating ferrous metal surfaces to degrease and clean thesurfaces and fonn thereon a thin, adherent phosphate coating.

It is well known in the art that aqueous acidic solutions based onalkali and/or ammonium orthophosphates and having a pH value in therange of about 3 to 6, may be used for simultaneously degreasing,cleaning and forming protective phosphate layers on iron or steelsurfaces. Generally, the phosphating solutions have been applied fromstationary immersion or spraying equipment and produce goodrust-proofing and/or paint-base coatings.

it has further been known to apply treating solutions of the above typefrom nonstationary systems, i.e., by means of steam injection or incombination with steam suction. In this manner of operation, thesolution is heated to about 140 C. and conveyed to a jet orifice underabout 5-9 atmospheres excess pressure (gauge pressure). The stream ofvery hot watersteam, which contains the solution components, is thensprayed onto the object which is to be treated. Although the coatingsproduced in this manner are comparable to those obtained in stationarysystems, frequently considerable variations in the quality of thecoating obtained will result. Moreover, in order to maintain the desiredconcentration of chemicals in the steam, special metering arrangementsare required, which necessitate constant control. Additionally, in manyplants there is no central supply of steam available so that an oil orgas-fed steam generator must be installed, which generator requiresconsiderable waiting time before it reaches operating condition and alsorequires constant maintenance. Further drawbacks which are encounteredin this method involve the fact that the process must be carried out atvery high temperatures, so that the steam emerging from the jets createsdifficult working conditions for operating personnel and, frequently,prevents any visual observation of the surfaces being treated. Thislatter aspect, in itself, may contribute considerably to thedifiiculties which are involved in maintaining uniform quality of thecoatings produced.

It is, therefore, an object of the present invention to provide a methodfor the pressure application of phosphate coatings, which methodoperates at appreciably lower temperatures than present methods usingsteam.

A further object of the present invention is to provide an improvedpressure application process for phosphate coatings, which processrequires appreciably less maintenance in operation while producingconsistently acceptable coating results.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows.

Pursuant to the above objects, the present invention includes a methodfor the treatment of metal surfaces which comprises spraying an aqueousacidic solution, containing alkali and/or ammonium orthophosphates andhaving a pH value of from about 3.0 to 5.5, and effecting the sprayingof this solution onto the metal surface to be treated at a pump pressureof from about to 75 atmospheres (gauge). The coatings produced by thismethod are found to provide an excellent base for the application ofpaints and similar protective films and although the method isparticularly adapted for the treatment of ferrous metal surfaces, suchas iron and steel, it may also be used on surfaces of zinc, aluminum,and their al loys.

More specifically, in the method of the present invention the aqueousacidic phosphating solutions used have a pH within the range of about 3to 5.5 and are based on alkali and/or ammonium orthophosphates.Preferably, these phosphates are present in the solution in amountswithin the range of about 2 to 10 grams per liter, calculated asNaH,P0,. it is to be appreciated that the compositions for use in thepresent invention may contain one or more of the alkali, i.e., sodium,potassium, lithium, cessium, or rhubidium,

orthophosphates or ammonium orthophosphates. Accordingly, forconvenience hereinafter, reference will be made to the use of alkaliorthophosphates, as including both the above-indicated alkaliorthophosphates and the ammonium orthophosphates. Additionally, althoughthe use of the monoalkali orthophosphates, such as monosodiumorthophosphate are preferred, the diand tri-alkali orthophosphates mayalso be used. Particularly with the use of these latter materials,phosphoric acid may be added to the solution to adjust the pH of thesolution to within the desired range of 3 to 5.5 and preferably withinthe range of about 3.5 to 4.

The treating solutions for use in the present method utilize theorthophosphates and it is preferred that any appreciable amounts ofcondensed alkali and/or ammonium phosphates in the solution be avoided.In this regard, it has been found that quantities of condensed alkaliand/or ammonium phosphates even as small as 3 percent by weight of theorthophosphates have a deliterious effect on the coatings which areformed. It is for this reason that the coating solutions of the presentinvention should be substantially free of any condensed alkali and/orammonium phosphates.

The phosphating solutions used in the method of the present inventiondesirably also contain a benzoate. The benzoate content of these coatingsolutions is desirably within the range of about 0.05 to 0.5 grams perliter, calculated as sodium benzoate, and preferably within the range ofabout 0.1 to 0.4 grams per liter. The benzoate may be incorporated inthe solution in the form of a benzoic acid, or as one of the solublesalts thereof, such as sodium benzoate.

in many instances, it is further desirable to effect the acceleration ofthe coating action of the solution by addition thereto of hydroxyl amineor its salts. Preferably, the hydroxyl amine is added as the salt, suchas hydroxyl amine acid sulfate, hydroxyl amine sulfate, hydroxyl aminehydrochloride, or the like, with the hydroxyl amine sulfate beingpreferred. Preferably, the hydroxyl amine content of the treating bathis from about 0.04 to 0.3 grams per liter, calculated as NH,OH.Additionally, the efficiency of the phosphating solution may be furtherimproved by the addition of a molybdate to the bath. Various solublemolybdate materials, particularly the alkali molybdates may be used. Themolybdate content of the bath is preferably from about 0.0l to 0.l gramsper liter, calculated as Na,MoO..

It has further been found that the cleaning and degreasing efficiency ofthe phosphating solutions may further be improved by incorporating asuitable surface-active agent in the solution. Although varioussurface-active agents have been found to be suitable, in many instancesthe nonionic surfaceactive agents, and particularly the derivatives ofthe polyoxyalkylenes are preferred. In some instances, however, variousanionic surface-active agents may also be used. Desirably, thesurface-active agents are present in the solution in amounts within therange of about 0.05 to l gram per liter.

in formulating the treating solutions for use in the method of thepresent invention, these solutions are desirably prepared from aconcentrate material, which concentrate may contain up to percent byweight of the monoalkali and/or monoammonium orthophosphate. Preferably,however, the concentrate composition will contain from about 60 to 97percent by weight of the monoalkali and/or monoammonium orthophosphate,from about 3 to 12 percent by weight of benzoate, calculated as sodiumbenzoate, up to about 7 percent by weight of hydroxyl amines or itssalts, calculated as Na oH, up to about 3 percent by weight of amolbdate, calculated as Na,MoO,, and up to about l2 percent by weight ofthe surface-active agent. This concentrate composition is then dissolvedin water to form the phosphating solutions for use in the presentmethod. Desirably, the concentrate composition is dissolved in water inamounts of from about 3 to 10 grams per liter, with amounts within therange of from about 4 to 6 grams per liter being preferred.

In applying the phosphating solutions, in accordance with the presentmethod, the solution is sprayed onto the metal surface to be treatedunder a pump pressure of from about to 75 atmospheres excess pressure.To obtain these high pressures, various pump systems may be used, as forexample, double-acting piston pumps with compressed-air drives; pistonpumps with electromotor drives; multistage rotary pumps; or gear pumps.With whatever type of pump is used, the treating solution is drawn bythe pump from a suitable storage tank, the temperature of the treatingsolution typically being within the range of about 50 to 80 C., althoughhigher and lower temperatures may be used in some instances. After thesolution has been compressed to the required high pressure, i.e., about10 to 75 atmospheres, it is led through one or more tubes or lanceswhich are provided, at their exit points, with appropriate spray jets.Although various types of jets may be used, in many instances it hasbeen found to be preferred to utilize flat jets, i.e. fishtail jets,having a capacity of from about 4 to 6 liters per minute.

In applying the coating solution, the spraying tubes or lances arepreferably used in such a manner that the jet is passed in a crosswisemotion at a distance of about l0 centimeters from the surface which isto be treated. Normally, the treatment should start at the top or upperend of the surface to be treated so as to minimize the quantities ofrunoff solution which come into contact with already treated surfaces.Where the solutions used contain surface-active agents, it has beenfound that frequently the surfaces will become grease-free and fullywetted after only about l0 to seconds treatment. Moreover, through theappropriate atomization of the treating solution into very finedroplets, which are sprayed at these high velocities, the solutionsreact very rapidly with the surface being treated and will, in veryshort time, form thin coherent iron oxide-iron phosphate layers.Generally, it has been found that when using only one spraying lance,depending upon the size, form and surface condition of the objects beingtreated, about 1 to 5 square meters of surface may be treated perminute.

In addition to pumping the actual treating solution, in carrying out themethod of the present invention, a preconcentrate containing up to asmuch as I00 grams per liter of the concentrate compositions describedabove, may also be used. In this method, these preconcentratecompositions will be admixed with water, in a mixing line, usingappropriate valve systems, to bring them to the desired treatingconcentration. In this manner, the need for bulky and cumbersomeequipment and storage tanks for the treating solution is avoided.

Once the application of the treating solutions, using the presentmethod, has been completed, it is generally desirable to rinse thetreated surfaces with water. This rinsing may be effected using eitherthe same spraying lances, which have been appropriately switched over towater, or alternatively, by means of a separate rinsing system. Wherethe treated objects are to be given a subsequent paint or similarcoating, they are desirably rinsed with deionized water so as to providesatisfactory paint stability. Generally, and particularly where thetreating solutions contain benzoate,'an after treatment with a corrosioninhibiting material, such as a solution containing chromic acid, is notnecessary. If desired, however, rinsing with chromic acid solutions orsimilar inhibiting materials may be utilized.

It has been found that where the treating solutions applied in themethod of the present invention contain a benzoate, there issubstantially no susceptability of the treated surfaces to rusting orother corrosion attack when they are allowed to dry in air, either withor without a subsequent water rinse. Accordingly, when such solutionsare used, neither oven drying or drying with compressed air isnecessary. In contrast, however, where the treating solutions usedcontain appreciable amounts of condensed phosphates, even with thebenzoates present, it is found that the treated surfaces have increasedcorrosion susceptability so that appreciable corrosion of the surfacestakes place when they are permitted to dry in air.

The method of the present invention has been found to be extremelyefficient, requiring low installation and chemical cost and involving asimple method of operation and a rapid startup and is, therefore,suitable for a wide variety of uses. This method has been found to beparticularly applicable in those installations where objects with largesurfaces, such as for example, agricultural machinery, buses, trucks ofbulky constructional components and the like are being produced.Additionally, the method is particularly applicable to operationswherein the number of objects treated per day is relatively small as forexample in the manufacture of special bodies, as well as in short runsor in repair and maintenance shops.

Although the method of the present invention is particularly suited forthe surface treatment of ferrous metal, e.g. iron and steel, the methodis also suitable for treating the surfaces of the zinc, aluminum, andtheir alloys. In the treatment of ferrous metal, the iron oxide-ironphosphate layer which is formed is generally from a blue to reddishshiny color, having a coating weight typically within the range of about0.3 to 0.8 grams per square meter. This coating has been found toprovide an excellent base for paint and similar organic coatings.Additionally, it is to be appreciated that although by use of the methodof the present invention, degreasing and cleaning of the metal surfacesis also effected, in those instances where the surfaces are covered withstrongly adhering impurities, it may be necessary to subject them toappropriate precleaning treatments before the utilization of the presentmethod.

In order that those skilled in the art may better understand the presentinvention and the manner in which it may be practiced, the followingspecific examples are given. In these examples, unless otherwiseindicated, parts and percents are by weight and temperatures are indegrees Centigrade. It is to be appreciated, however, that these aremerely exemplary of the present invention and are not to be taken as alimitation thereof.

EXAMPLE I An aqueous treating solution was formulated containing thefollowing components in the amounts indicated:

Phosphoric acid was added to the solution to adjust the pH to 3.5 andthe solution was heated to C. The solution was then compressed to about30 atmospheres excess pressure by means of a compressed air-drivenhigh-pressure pump and was then sprayed onto grease-coated steel sheetsfor one minute. The temperature of the solution as it emerged from thespraying jets was about 60 C., and the distance between the jet and thesheet being treated was about 10 centimeters.

Thereafter, the treated sheets were rinsed by spraying with water for 30seconds at 20 C.; sprayed for 30 seconds with a 55 C. solutioncontaining 0.16 grams per liter CrO, and 0.03 grams per liter Cr(IlI)ions; rinsed by spraying with deionized water and then dried for l0minutes in a forced air circulating oven at I10 C. The thus'treatedsheets were coated with a deep-blue, uniform iron oxide-iron phosphatelayer.

The thus-treated surfaces were then spray painted with a polyacrylatebase paint, the thickness of the applied paint film being about 26 pm.The painted sheets were then subjected to the standard salt-spray testin accordance with the procedure ASTM Bl 17-64. After 168 hours in thetest, there was found to be a corrosion creepage of only 3 millimetersfrom the scratch line.

EXAMPLE 2 By way of comparison, a treating solution was formulatedcontaining the following components in the amounts indicated:

ComponenLs Grams per Liter NaH,PO,(water-free) 2.8 Na,H,P,O,(water-free)L6 Phosphoric acid (P,O, content 725) 0,3 Nonionic surface active agent(as in Exam le I) 0.3

The pH of this solution was 4. Identical steel sheets, as used inexample 1 were then treated with the solution in a conventionalstationary four-zone spraying installation, the treating solution beingsprayed for 2 minutes at 60 C. under a pressure of 1.5 atmospheres. Thisspray treatment with the treating solution was then repeated followingwhich the sheets were rinsed with water, chromic-acid-containingsolutions, deionized water, and dried, in the same manner as set forthin example 1. The iron oxide-iron phosphate coatings produced on thesheets were light blue in color.

The treated steel sheets were then painted, as in example I, andsubjected to the same salt-spray test. In this instance, however, acorrosion creepage of three millimeters from the scratch line wasproduced after only 92 hours in the test.

EXAMPLE 3 In an automobile production plant, car bodies were treatedusing the high-pressure application method of the present invention toform a paint-base coating. In these experiments, a high-pressurespraying device was used in conjunction with a double-acting piston pumpdriven by compressed air at about 3 atmospheres over pressure. Thephosphatizing solutions used were drawn into the pump from a storagetank, compressed to about 30 atmospheres excess pressure and were thensprayed onto the surfaces to be treated using two spraying lances. Eachof the lances was provided with fishtail jets having an equivalentdiameter of about 0.9 millimeters and an angle of the opening of thefishtailjets of about 65.

Two different solutions, designated A and B, were used for theexperiments, which solutions contained the following components in theamounts indicated:

Both of these solutions were adjusted with phosphoric acid to a pH ofabout 4 and were then heated to about 80 C. by means of immersionheaters. The treating solutions were sprayed onto the objects with a jethaving a spray capacity of about 4 liters per minute. The temperature ofthe sprayed solution was about 60 C. and the distance between the jetsand metal surfaces was about to 20 centimeters. The spraying was startedon the upper portion of the body and progressed slowly downwardly with adouble crosswise motion. About one to two square meters of surface areawere treated per minute. After about 10 minutes, the cleaning andprotective layer formation was completed over the entire body.Thereafter, the bodies were sprayed with water and the residual waterfilm was allowed to dry in air, about 3 to 5 minutes being required toeffect complete drying. On those bodies which had been sprayed withsolution A, it was found that a very heavy rust layer appeared withinabout 30 seconds after drying. In contrast, on the bodies on whichsolution B had been used, there was no evidence of rust or othercorrosion after the drying in air.

The above experiments were again repeated with the exception thatfollowing the water rinsing, the bodies were after rinsed with asolution, at 50 C., made up with deionized water and containing 0.16grams per liter CrO and 0.03 grams per liter Cr(lll) ions. In thisinstance, it was found that the bodies which had been treated withsolution A, as well as those treated with solution B were dried in airwithout any sign of COlTOSlOn.

EXAMPLE 4 Grease-coated steel sheets were treated for one minute withthe solutions A and B, as described in example 3 above, using a pumppressure of about 30 atmospheres and a solution temperature of 60 C. Thetreated sheets were then rinsed with water, after-rinsed with deionizedwater and dried. A polyacrylate base paint was then applied by sprayingto form a paint thickness of about 25 t. These sheets were thensubjected to the standard salt-spray test, according to ASTM B l l764.The sheets treated with solution A SHOWED A corrosion creepage of 3millimeters after 96 hours in test while the sheets treated withsolution 8 showed a corrosion creepage of 3 millimeters after I20 hoursin tests.

While there have been described various embodiments of the invention,the compositions and methods described are not intended to be understoodas limiting the scope of the invention, as it is realized that changesthercwithin are possible and it is intended that each element recited inany of the following claims is to be understood as referring to allequivalent elements for accomplishing substantially the same results insubstantially the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. In the process, wherein a phosphate coating is formed on a metalsurface by the spray application of an aqueous acidic phosphatizingsolution, the improvement which comprises formulating the phosphatingsolution with alkali and/or ammonium orthophosphates, maintaining thesolution pH within the range of about 3.0 to 5.5 and spraying thesolution onto the metal surface at a pump pressure of from about l0 to75 atmospheres excess pressure.

2. The method as claimed in claim I wherein the phosphatizing solutionused also contains from about 0.05 to 0.5 grams per liter benzoate,calculated as sodium benzoate.

3. The method as claimed in claim 2 wherein the solution contains fromabout 0.1 to 0.4 grams per liter benzoate.

4. The method as claimed in claim 2 wherein the phosphatizing solutionalso contains an accelerator selected from hydroxyl amine and hydroxylamine salts, in an amount of from about 0.04 to 0.3 grams per liter,calculated as NH,OH.

5. The method as claimed in claim 4 wherein the phosphatizing solutionalso contains from about 0.01 to 0.l gram per liter molybdate,calculated as Na,MoO,.

6. The method as claimed in claim 5 wherein the phosphatizing solutionalso contains from about 0.05 to l gram per liter of a surface-activeagent.

7. A concentrate composition, suitable for the preparation of an aqueousacidic phosphating solution for use in the method of claim 1, whichconcentrate composition comprises from about 60 to 97 percent by weightofa monoalkali and/or monoammonium orthophosphate, from about 3 to l2percent by weight benzoate, calculated as sodium benzoate, anaccelerator selected from hydroxyamine and hydroxylamine salts, in anamount up to about 7 percent by weight, calculated as N H OH, up toabout 3 percent by weight molyhdate, calculated as Na Moo and up toabout 12 percent by weight of a surface-active agent.

i i i

2. The method as claimed in claim 1 wherein the phosphatizing solutionused also contains from about 0.05 to 0.5 grams per liter benzoate,calculated as sodium benzoate.
 3. The method as claimed in claim 2wherein the solution contains from about 0.1 to 0.4 grams per literbenzoate.
 4. The method as claimed in claim 2 wherein the phosphatizingsolution also contains an accelerator selected from hydroxyl amine andhydroxyl amine salts, in an amount of from about 0.04 to 0.3 grams perliter, calculated as NH2OH.
 5. The method as claimed in claim 4 whereinthe phosphatizing solution also contains from about 0.01 to 0.1 gram perliter molybdate, calculated as Na2MoO4.
 6. The method as claimed inclaim 5 wherein the phosphatizing solution also contains from about 0.05to 1 gram per liter of a surface-active agent.
 7. A concentratecomposition, suitable for the preparation of an aqueous acidicphosphating solution for use in the method of claim 1, which concentratecomposition comprises from about 60 to 97 percent by weight of amonoalkali and/or monoammonium orthophosphate, from about 3 to 12percent by weight benzoate, calculated as sodium benzoate, anaccelerator selected from hydroxyamine and hydroxylamine salts, in anamount up to about 7 percent by weight, calculated as NH2OH, up to about3 percent by weight molybdate, calculated as Na2MoO4 and up to about 12percent by weight of a surface-active agent.